The present invention relates to a pneumatic tire provided with a plurality of main grooves extended in a tire circumferential direction on the tread surface. More particularly, the present invention relates to a pneumatic tire capable of controlling uneven wear occurring in the vicinity of the main grooves.
Generally, on the tread surface of the pneumatic tire, a single or plural curvature radius (tread radius), as a curvature, in a tire meridian direction is imparted based on the inner surface shape of a die mold. Meanwhile, in the pneumatic tire, the curvature radius on the tread surface tends to be changed during pressurization due to its inner structures such as a belt layer buried inside the tread portion, tread rubber and the like. When a change in curvature radius on the tread surface occurs in the above manner due to inflation, this change is absorbed by the groove portion, resulting in the occurrence of a phenomenon such as bending of the tread portion at the groove bottom as a boundary. Particularly, in a rib tire provided with a plurality of main grooves extended in the tire circumferential direction, the above bending phenomenon is significant.
As described above, when the tread portion is bent at the groove bottom as a boundary, the edge portion of the rib adjacent to the groove does not match the specified curvature radius of the tread surface, and the ground-contacting pressure on the rib edge portion is significantly changed. As a result, uneven wear occurs with the rib edge portion as a starting point, resulting in a growth of railway wear.
The object of the present invention is to provide a pneumatic tire capable of effectively controlling uneven wear occurring in the vicinity of a main groove even in the case where the groove width of the main groove is narrowed due to a change in the tread radius during inflation.
A pneumatic tire of the present invention for achieving the foregoing object is a pneumatic tire provided with a plurality of main grooves extended in a tire circumferential direction on the tread surface, characterized in that, with regard to some of the main grooves among the foregoing plurality of main grooves, which have zigzag shapes swinging in a tire width direction and also whose groove width is narrowed during inflation, in a bent portion near the shoulder, an inclination angle of a groove wall near the shoulder with respect to the tread surface is made larger than an inclination angle of a groove wall near the center, in a bent portion near the center, the inclination angle of the groove wall near the center with respect to the tread surface is made larger than the inclination angle of the groove wall near the shoulder, and regions where the inclination angles of such opposed groove walls are different from each other are arranged uncontinuously in the tire circumferential direction.
As a result of an intensive research on an occurrence mechanism of uneven wear such as railway wear, the inventors of the present application found out that, in the case where the zigzag-shaped main groove having the groove width narrowed due to the change in tread radius during inflation, a ground-contacting pressure on an edge portion having a concave shape in a bent portion of the main groove is locally increased, and that the uneven wear occurs with that portion as a starting point.
In this connection, as described above, with regard to the zigzag-shaped main groove having the groove width narrowed due to the change in tread radius during inflation, in the bent portion near the shoulder, the inclination angle of the groove wall near the shoulder is made relatively large, and in the bent portion near the center, the inclination angle of the groove wall near the center is made relatively large. In doing so, the rigidity of these regions is reduced, and the increase of the ground-contacting pressure on a rib edge portion is controlled. Hence, the uneven wear such as railway wear occurring in the vicinity of the main groove can be effectively controlled.
More specifically, with regard to the main groove having the groove width narrowed during inflation, it is preferable that, in the bent portion near the shoulder, while the inclination angle of the groove wall near the shoulder with respect to the tread surface is made larger than 90xc2x0, the inclination angle of the groove wall near the center is made smaller than 90xc2x0, and that, in the bent portion near the center, while the inclination angle of the groove wall near the center with respect to the tread surface is made larger than 90xc2x0, the inclination angle of the groove wall near the shoulder is made smaller than 90xc2x0. As described above, by opposing the groove wall having the inclination angle smaller than 90xc2x0 and the overhang-shaped groove wall having the inclination angle larger than 90xc2x0, the rigidity of the both sides of the main groove can be effectively adjusted.
Moreover, in the event of arranging uncontinuously the regions where the inclination angles of the opposed groove walls are different from each other (rigidity adjustment portions), for a zigzag pitch Z in the tire circumferential direction of the main groove having the groove width narrowed during the inflation, the foregoing rigidity adjustment portions should be provided in an area within 0.15 Z on both sides of the groove bent portion.